fs: make fs.access() flags read only

These flags were defined as constants, but could be
overwritten when exported from fs. This commit exports
the flags as read only properties of fs.

PR-URL: https://github.com/iojs/io.js/pull/507


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>